There have been proposed various methods for producing lithium hexafluorophosphate. For example, there is a method (Patent Publication 1) of reacting a solid lithium fluoride with a gaseous phosphorus pentafluoride with no solvent. In this method, a film of the reaction product is formed on the surface of lithium fluoride, thereby the reaction does not proceed completely, and the unreacted lithium fluoride remains. Furthermore, there are a method (Patent Publication 2) of reacting lithium fluoride with HF in phosphorus pentachloride, a method (Patent Publication 3) of reacting phosphorus trichloride, elemental chloride and HF, and the like. In each of these, phosphorus trichloride or phosphorus pentachloride, which is highly hygroscopic, is reacted directly with HF. Therefore, moisture contained in these and moisture absorbed from the air upon charging are introduced, and a lithium oxyfluoride, which is easily hydrolyzed, is generated in and mixed with the product. Even if one tries to use it as the electrolyte of a lithium ion battery, it is hydrolyzed by a trace amount of, moisture in the electrolyte solution to generate an acidic substance. Since the electrolyte solution is damaged thereby, there is a problem that it cannot be used as the electrolyte of a lithium ion battery. Furthermore, there is a method (Patent Publication 4) of reacting lithium fluoride with phosphorus pentafluoride in an organic solvent. However, since phosphorus pentafluoride is a gas, a cylinder is necessary for its handling. Therefore, the production of phosphorus pentafluoride is complicated. Since gas is handled, it involves danger, and an expert knowledge is necessary. Furthermore, it is necessary to purify a phosphorus pentafluoride used to have a high purity. Therefore, there is a problem that the cost becomes high to greatly affect the price.
Furthermore, there is a method (Patent Publication 5) of reacting a gaseous phosphorus pentafluoride with lithium fluoride that is dissolved when anhydrous hydrogen fluoride is used as a solvent. In this method, handling is difficult since anhydrous hydrogen fluoride, which is high in vapor pressure, is used as the solvent.
As mentioned above, each of the conventional methods was not necessarily satisfactory in terms of reaction yield, easiness of control of reaction, purity of the obtained product, and the like.    Patent Publication 1: Japanese Patent Application Publication 64-72901    Patent Publication 2: Japanese Patent Application Publication 10-72207    Patent Publication 3: Japanese Patent Application Publication 10-81505    Patent Publication 4: Japanese Patent Application Publication 9-165210    Patent Publication 5: Japanese Patent Application Publication 6-56413